Low carbon ferrous alloy containing chromium

ABSTRACT

THE ALLOW CONTAINS CR, 13 TO 25%, TI 2 TO 7%, AND OPTIONALLY ONE OR MORE OF THE FOLLOWING METALS: AL 0 TO 6%, V 2 TO 7%, SI 2 TO 7%, MO 0 TO 3%, RARE EARTH METALS 0 TO 0.5% EACH. THE BALANCE IS FE WITH AT MOST 0.005% C. THE PROPERTIES OF THE ALLOY ARE IMPROVED BY SOLUTION HEAT-TREATMENT FOLLOWED BY HOT-WORKING.

United States Patent M rm. (:1. 6m 39/14 U.S. Cl. 75-126 D 1 Claim ABSTRACT OF THE DISCLOSURE The alloy contains Cr, 13 to 25%, Ti 2 to 7%, and optionally one or more of the following metals: Al 0 to 6%, V 2 to 7%, Si 2 to 7%, Mo 0 to 3%, rare earth metals 0 to 0.5% each. The balance is Fe with at most 0.005% C. The properties of the alloy are improved by solution heat-treatment followed by hot-working.

The invention concerns metal alloys, and more particularly metal alloys intended for use with the high temperatures, for instance approximately 700 C., to which materials are subjected when used as a protection sheath for nuclear fuel used in fast neutron nuclear reactors.

Certain austenitic stainless steels have properties which make them suitable for use in making protection sheaths for nuclear fuel used in nuclear reactors of the thermal type. In this connection, it is known that the crystal structure of these steels, i.e. the faced centred cubic system, contributes to their high mechanical properties.

It has however been observed that the austenitic steels mentioned above sometimes have a tendency to brittle fracture; one of the reasons for this tendency may in particular arise from the effects of the irradiation of these steels by a neutron flux having a density of fast neutrons which is considerably greater than that which is met with in thermal nuclear reactors. This disadvantage has the result of restricting the use of stainless austenitic steels, and in particular of making them unsuitable for applications in which they are subjected to such a flux.

To the knowledge of the inventors, up to the present, it has not been possible to find means for overcoming this disadvantage in a satisfactory manner. A metallic alloy would be desirable which, while conserving good resistance to corrosion and excellent mechanical properties, these being the result of a hardening of solid solution and precipitation hardening, has distinctly less sensitivity to fragile breaking.

The metallic alloy forming the subject of the invention contains, by weight:

balance Fe containing no more than 0.005 carbon and incidental impurities.

It has been found that alloys fulfilling the above mentioned conditions retain satisfactory properties at high temperatures, as well as low sensitivity to brittle fracture, as a result of the body centred cubic structure (being different in this respect from that of stainless austenitic steels).

The properties of these metallic alloys are improved if in addition to the elements already indicated, they contain one or more of the following elements:

3,719,475 Patented Mar. 6, 1973 Percent Aluminium From 0 to 6 Vanadium From 2 to 7 Silicon From 2 to 7 Molybdenum From 0 to 3 One or more rare earth metals from 0 to 0.5%.

The above alloys may be prepared in a normal crucible (fritted magnesia) in accordance with the customary method, cast, and cooled, preferably under vacuum or under an inert or neutral atmosphere. No particular problem is posed by their method of preparation.

Moreover, the invention also aims at producing a method of treatment which, applied to the above-mentioned alloys, makes it possible to improve the properties still further. In accordance with this method, after casting and cooling, these alloys are subjected to at least one operation comprising heating to a temperature which is sufiicient (most frequently 1,000 C.) to put in solution all the solid precipitates which may be present, the heating being followed, preferably at temperature which is as near as possible to the heating temperature, by between 20% and 60% hot working, this thermomechanical treatment leading to a fresh distribution of the hardening precipitation.

The concentration ranges in the basic Fe-Cr-Ti alloy are chosen for the following resaons. If the Ti content is below 2% there is insufficient precipitation hardening, whereas if the content is above 7%, the elongation at fracture is below 8%. A Cr content below 13% gives insufficient resistance to oxidation, whereas at concentrations above 25%, brittleness occurs, due to the appearance of the delta phase, at 500700 C.

The optimal additional elements improve the basic alloy by increasing the mechanical strength at service tem peratures due to intermetallic precipitates, refining the structure when submitted to thermo-mechanical treatment, and allowing easier forming of the alloy.

EXAMPLE 1 A ferritic alloy, having the composition 13% Cr, 5% Ti, balance Fe, was extruded at 1050" C. The following properties were obtained at 700 C.:

Yield strength "kg/mm? 15.9

Ultimate tensile strength kg./mm. 19.3

Elongation percent 49.5

EXAMPLE 2 A ferritic alloy, having the composition given below: 13% Cr, 5% Ti, 2% V, 0.2% A1, 0.1% Ce, balance Fe, was prepared, cast, cooled and then aged for three days at a temperature of 700 C. It then had the following mechanical properties measured at 700 C. at a traction speed of 0.5 per min.:

Yield strength kg./mm. 16.2 Ultimate tensile strength kg./rnm. 18.2 Elongation percent 27 Subjected to thermo-mechanical treatment including a first mechanical hammering of 40% deformation after heating at 1100" C., then a second hammering of 30% deformation after heating at 900 C. this same alloy had, at 700 C., the following properties:

Yield strength "kg/mm?" 11 Ultimate tensile strength kg./mm. 14 Elongation percent The chemical composition (small effective fast-neutron absorption cross-section, compatibility with the fuel taken in the form of carbides and with the heat carrying fluid), crystalline structure (body centred cubic), and mechanical properties at high temperatures (between those of austenitic steels and of ferritic steels) makes this ferritic References Cited alloy a possible base material for the preparation and UNITED STATES PATENTS manufacture of linings of fast neutron reactors.

We claim: 1,493,191 5/ 1924 Golyer 75126 D 1. A ferritic alloy containing by Weight: zero to 0.005% 1,508,032 9/1924 Smlth 75 126 carbon; 13% to 25% chromium; 2% to 7% titanium; at 5 g f f g It t1 ltdf th 't' of: ans

e35 me 3 Se 6 mm B group 6011515 3,027,252 3/1962 McGurty 75 124 aluminum from zero to 6% vanadium from 2% to 7% HYLAND BIZOT, Primary Examiner silicon from 2% to 7% 10 molybdenum from zero to 3%, and US. Cl. X.R.

at least one rare earth metal from zero to 0.5% per rare earth metal; the balance being iron and incidental impurities. 

